Abstract Exposure to ionizing radiation is often fatal due to acute radiation syndromes (ARS) manifested as Gastrointestinal-ARS (GI-ARS) and Hematopoietic-ARS (H-ARS). Delayed effects of acute radiation exposure (DEARE) lead to multi-organ dysfunction syndrome (MODS). A common denominator of radiation induced multi-organ failure is due to damage to endothelial cells (ECs) and lymphatic ECs, resulting in leakiness, coagulopathy and inflammation, setting up stage for infection, sclerosis and tumorigenesis. The molecular basis of radiation-induced EC dysfunction is not well understood. Our goal is to capitalize on the regenerative function of ECs by intravenously transplanting readily-available, off-the- shelf, allogeneic human ECs to mitigate ARS, DEARE and MODS. Our central hypothesis is that radiation damaged blood vessel and lymphatic ECs become dysfunctional and fail to perform their vascular functions or supply the instructive signals required to promote tissue healing thereby leading to ARS and DEARE. We propose that transplantation of normal pro-regenerative ECs a day or days after radiation can rescue the multi-organ defects of radiation-injured ECs and promote scar-free healing. We have shown that tissue-specific ECs by producing angiocrine growth factors orchestrate the repair of injured organs without fibrosis. Intravenous transplantation of human ECs restores hematopoietic recovery in sublethally irradiated rodents and lethally irradiated pigtail macaque non-human primates (NHP) without fibrosis or tumorigenesis. The Rationale for the proposed experiments is that if we know how to efficiently generate abundant off-the-shelf GMP-grade human umbilical vein ECs (HUVECs) as a ?generic allogeneic vascular graft?, we will use NHP large animal model radiation models to determine the pharmacokinetics of HUVEC transplantation to use them as a definitive or intermediary radiation countermeasure to support organ repair post-radiation. We will test this hypothesis by addressing these Aims: 1) Manufacture of abundant functional clinical-grade master cell banks of monkey ECs (MUVECs) and human (HUVECs) for intravenous transplantation. 2) Identify the critical parameters for allogeneic/xenogeneic MUVEC and HUVEC transplantation into recipient mice to mitigate post-irradiation H-ARS and GI-ARS injury without provoking fibrosis..3) Employ pigtail macaque NHP radiation models to determine the scheduling, safety and efficacy of transplanting MUVECs and HUVECs to rejuvenate vascular niche for multi-organ repair without scarring. Completion of the proposed studies will enable therapeutic use of allogeneic off-the-shelf ?human ECs? that transiently home to the disrupted vascular beds of irradiated organs restoring angiogenesis and vascular niche functions promoting organ repair, scarring. The success of these studies will provide for a readily available medical counter measure (MCM) for the treatment of acute and chronic radiation syndromes preventing life threatening complications. 1